1. Field of the Invention
This invention relates to a method of fabricating thallium-containing superconducting oxides (and especially the Ti:Ba:Ca:Cu 2223 phase oxides) through use of an optimized sintering/cooling schedule which gives resultant oxides which are both relatively pure (greater than 95%) and which exhibit substantial mechanical strength and structural integrity. More particularly, it is concerned with such a fabrication method, wherein a compressed body formed of non-superconducting starting oxides is subjected to a sintering process wherein the temperature of the body is gradually raised to maximum level of from about 850.degree.-930.degree. C., and is maintained at this temperature for a period of at least about 24 hours; thereupon, the body is cooled, preferably by a staged cooling process, in order to yield the final superconducting oxides.
2. Description of the Prior Art
Superconductivity refers to that special state of a material where its resistance to electrical current flow suddenly and completely disappears when its temperature is lowered. Below this onset or critical temperature T.sub.c, a characteristic of the material, the electrical resistance does not merely drop to a low level but it vanishes entirely. Only a very limited list of materials exhibit such a state. The discovery of the first superconductor occurred in 1911. Heike Kammerlingh Onnes discovered that mercury lost all detectable resistance at a temperature just 4.degree. above absolute zero.
A superconductor also exhibits perfect diamagnetism below its critical temperature, i.e., it expels all magnetic field lines from its interior by producing an opposing magnetic field from a current flowing on its surface. As a consequence of the perfect diamagnetism of superconductors, they can be used to produce magnetic levitation as envisioned in high speed transport systems of the future, where magnetic repulsion is used to counter gravity. The perfect diamagnetism property of superconductors is called the Meissner effect after its discoverer.
One class of superconducting oxides heretofore examined are the high T.sub.c Tl-Ba-Ca-Cu-O superconducting oxides, especially the 2223 oxides. The conventional method of preparing such oxides is to heat the primary oxides (e.g., Tl.sub.2 O.sub.3, BaO, CaO and CuO) at temperatures close to the melting point of the thallium oxide for a short period of time, usually 5-60 minutes. Such short sintering is done to avoid sublimation of Tl, but the period is too short for the chemical components to react sufficiently. While these superconductors show considerable promise, they are plagued by a number of problems attributable to the method of fabrication thereof. For example, the conventional fabrication method usually results in a majority of 2212 superconducting phase and only a smaller amount of the higher T.sub.c (greater than 120.degree. K.) 2223 phase. Moreover, it is sometimes difficult to fabricate large volumes of the oxides using the conventional techniques, owing to the fact that thallium may sublimate during sintering or escape of water vapor may cause cracking. Accordingly, conventional techniques do not provide high quality, essentially single phase thallium-based superconducting oxides. Morosin et al., "Crystal Structure of TlCa.sub.2 Ba.sub.2 Cu.sub.3 O.sub.9 ", Physica C 156 (1988), p. 587-591 describe the production of single crystal superconductors, i.e., crystals consisting of identical unit cells all of which are aligned along an identical crystal axis. Such materials are to be contrasted with polycrystalline superconductors wherein the unit cells are not aligned along a common crystal axis. Thus, polycrystalline superconductors may be viewed as a composite of randomly oriented grains each made up of single crystals. As can be appreciated, virtually all bulk superconductors are polycrystalline in character, whereas single crystal materials are generally limited to thin film systems.
It has also been known to produce mixed phase thallium superconductors containing a majority of phases such as 2212 and 2201, with only a relatively small proportion of the most desirable, high T.sub.c 2223 phase. These mixed phase superconductors inevitably exhibit lower critical temperatures and are therefore deficient.